Lubomir Spanhel

Bio: Lubomir Spanhel is an academic researcher. The author has contributed to research in topics: Semiconductor & Charge carrier. The author has an hindex of 7, co-authored 8 publications receiving 3159 citations.

Photochemistry of colloidal semiconductors. 20. Surface modification and stability of strong luminescing CdS particles

Abstract: The preparation of CdS sols with a mean diameter between 40 and 60 A and a relatively narrow size distribution is described. The colloids could be separated as solids, which in turn could be redissolved to give solutions of some 10/sup -2/ M. Activation of the particles by a cadmium hydroxide precipitate yielded fluorescing samples with a quantum yield exceeding 50%. The blue or green fluorescence occurred close to the band gap energy, which depended on the size of the particles. Violet fluorescing samples of activated ZnS-CdS co-colloids were also prepared. Photoanodic corrosion measurements showed that the activated CdS colloid was 2000 times more stable than the nonactivated one. Under laser illumination the particles became a little larger which is ascribed to an acceleration of Ostwald ripening due to local heating. Extremely intense laser light decomposed the particles into Cd + S.

Semiconductor clusters in the sol-gel process: quantized aggregation, gelation, and crystal growth in concentrated zinc oxide colloids

Abstract: A new synthesis of ZnO wurtzite clusters (crystallite sizes 3-6 nm) is presented. This novel approach, employing ultrasound, allows one to produce relatively highly concentrated 0.1 M Q-ZnO colloids within a few minutes. These colloids remains in a dispersed state for weeks. They can be further concentrated into stable syruplike liquids (molarities of ∼1 M). Under the externe concentration conditions (10 M Q-ZnO colloids) employed, ZnO alcogels are formed, and self-induced crystal growth occurs in the alcogels (sizes ranging between 1 and 5 mm)

Photochemistry of semiconductor colloids. 22. Electron ejection from illuminated cadmium sulfide into attached titanium and zinc oxide particles

Abstract: Colloidal solutions of CdS containing colloidal TiO, or ZnO were illuminated with visible light. The fluorescence of CdS (decay time -50 ns) was quenched by Ti02, several Ti02 particles being required per CdS particle. The rate of photoanodic corrosion in aerated solution was drastically increased in the presence of Ti02 In deaerated CdS solutions containing methanol and Cd2+ ions, cadmium metal was formed when Ti02 was present. Methyl viologen was reduced with a quantum yield of close to one, while it reacted about ten times more slowly in the absence of Ti02 These effects are explained in terms of improved charge separation by rapid electron injection from illuminated CdS into the conduction band of attached Ti02 particles. Electron injection into ZnO was less efficient and occurred only in the case of Q-CdS particles (very small particles having a greater band gap). The injected electrons caused a blue shift of the absorption threshold of ZnO. The photocatalytic action of colloidal or suspended semicon- ductor particles is based on the generation of electrons and positive holes which rapidly move to the surface of the particles and initiate redox processes. The efficiency of charge separation is often increased by contacting the semiconductor particle with a metal or another semiconductor. Typical examples are platinized ti- tanium dioxide' and cadmium sulfide2 as well as Ru02-covered Ti02.3 Serpone et al. reported a few years ago that H2 was formed from H2S on CdS powder illuminated with visible light in aqueous solution and that the yield was slightly increased in the presence of Ti02 powder! The effect was explained by an improved charge separation due to electron transfer from the illuminated CdS particles into the conduction band of the Ti02 particles. The increase in yield was only 20%, Le., little above the increase which could be explained by more efficient light absorption of CdS due to the increased internal light scattering by the TiO, additive. In the present paper, experiments with transparent colloidal solutions of CdS containing colloidal Ti02 or ZnO as additives are described. Efficient electron injection from the excited CdS part of the "sandwich" colloids to the Ti02 or ZnO part was observed with three methods of observation: (1) With use of a CdS colloid that fluoresces with a high quantum yield, the quenching of the fluorescence by added TiO, or ZnO was studied. (2) Redox processes, such as the reduction of excess Cd2+ ions and of methyl viologen and the photoanodic dissolution of CdS, were initiated by visible light illumination and the influence of added Ti02 investigated. (3) In the case of ZnO as additive, the electron injection was accompanied by the typical changes in the absorption spectrum of ZnO which have recently been observed in other experiments on the deposition of excess electrons on small semiconductor particles.s-6

Detection of the intermediates of colloidal TiO2-catalysed photoreactions

Abstract: Electrons and positive holes have been produced in colloidal TiO2 particles upon illumination with near-u.v. light. In the absence of adsorbed reactants, the charge carriers recombine. In the presence of an adsorbed reactant for holes [such as SCN– or poly(vinyl alcohol)], an excess of long-lived electrons remains on the particles and finally reacts with a reactant in the bulk solution. In the presence of an adsorbed reactant for electrons (such as a platinum deposit or methyl viologen), an excess of long-lived holes remains and finally reacts with an oxidizable compound. The processes observed include the reduction of water to form H2, the reduction of tetranitromethane and halothane and the oxidation of Br– and various organic compounds. Methyl viologen, MV2+, was found to be reduced to the radical MV˙+ and also to be oxidized, to a lesser extent, to form a fluorescing compound. The absorption spectra of the long-lived excess of electrons and positive holes trapped at the interface were measured using flash photolysis.

Synthesis of porous quantum-size CdS membranes: Photoluminescence phase shift and demodulation measurements

Abstract: Optically transparent CdS membranes have been synthesized. Colloidal Q-CdS particles (particle diameter d{sub p} < 40 {angstrom}) carrying short phosphate chains and excess Cd{sup 2+} ions have been directly converted into a microporous membrane form. By controlling ionic strength and particle concentrations, one can link self-organized fusion-free aggregates and avoid formation of powder-like flocculation products. The resultant unsupported membranes exhibit different mechanical properties (e.g., rigidity, delayed elasticity, and solubility) in the presence of water depending upon the preparation method employed. The membrane form of CdS has distinctively different photophysical properties than the precursor colloidal form. Conversion of weakly red luminescent colloids (broad band at 700 nm) into membranes activated an intense room temperature band edge luminescence (BEL) (narrow bands between 450 and 500 nm) attributed to the recombination of excitons and/or shallowly trapped electron/hole pairs.

Chemistry and properties of nanocrystals of different shapes.

Abstract: The interest in nanoscale materials stems from the fact that new properties are acquired at this length scale and, equally important, that these properties * To whom correspondence should be addressed. Phone, 404-8940292; fax, 404-894-0294; e-mail, mostafa.el-sayed@ chemistry.gatech.edu. † Case Western Reserve UniversitysMillis 2258. ‡ Phone, 216-368-5918; fax, 216-368-3006; e-mail, burda@case.edu. § Georgia Institute of Technology. 1025 Chem. Rev. 2005, 105, 1025−1102

Semiconductor-based Photocatalytic Hydrogen Generation

Abstract: 2.3. Evaluation of Photocatalytic Water Splitting 6507 2.3.1. Photocatalytic Activity 6507 2.3.2. Photocatalytic Stability 6507 3. UV-Active Photocatalysts for Water Splitting 6507 3.1. d0 Metal Oxide Photocatalyts 6507 3.1.1. Ti-, Zr-Based Oxides 6507 3.1.2. Nb-, Ta-Based Oxides 6514 3.1.3. W-, Mo-Based Oxides 6517 3.1.4. Other d0 Metal Oxides 6518 3.2. d10 Metal Oxide Photocatalyts 6518 3.3. f0 Metal Oxide Photocatalysts 6518 3.4. Nonoxide Photocatalysts 6518 4. Approaches to Modifying the Electronic Band Structure for Visible-Light Harvesting 6519

Quantum dots versus organic dyes as fluorescent labels

Abstract: Suitable labels are at the core of Luminescence and fluorescence imaging and sensing. One of the most exciting, yet also controversial, advances in label technology is the emerging development of quantum dots (QDs)--inorganic nanocrystals with unique optical and chemical properties but complicated surface chemistry--as in vitro and in vivo fluorophores. Here we compare and evaluate the differences in physicochemical properties of common fluorescent labels, focusing on traditional organic dyes and QDs. Our aim is to provide a better understanding of the advantages and limitations of both classes of chromophores, to facilitate label choice and to address future challenges in the rational design and manipulation of QD labels.